This invention relates to heat treatment of turbine components and specifically, to a heat treatment schedule for achieving different properties at different locations in a nickel base superalloy turbine rotor disk.
Generally, it is known that differential heat treatment of an object can be employed to impart different properties at different locations. However, while such treatment works well on cylindrical objects, it is difficult to implement on more complex shapes such as found in turbine wheels or disks.
Alloy 706 is a nickel-based superalloy used for high temperature applications in gas turbines. This alloy can be used in connection with two heat treatment conditions identified by the inventor of the alloy (International Nickel Company) in the 1960's. The two known heat treatment processes are as follows:
Heat Treatment A.
Solution treatment at 1700–1850° F. for a time commensurate with section size, then air cool;
Stabilization Treatment at 1550° F. for three hours, then air cool; and
Precipitation Treatment at 1325° F. for 8 hr, then furnace cool at 100° F./hr to 1150° F./8 hr, then air cool.
Heat Treatment B.
Solution Treatment at 1700–1850° F. for a time commensurate with section size, then air cool; and
Precipitation Treatment at 1350° F. for 8 hr, then furnace cool at 100° F./hr to 1150° F./hr, air cool.
Heat Treatment A is typically recommended for optimum creep and rupture properties, while heat treatment B is typically recommended for applications requiring high tensile strength. A turbine rotor wheel or disk requires high tensile strength at low and intermediate temperatures (<700° F.) in some locations of the forging (e.g., near the bore and bolt holes) and optimum creep behavior in other parts (e.g., near the radially outer end). However, the OD of the part which is attached to the turbine blades, is at higher temperature during operation. If Heat Treatment A is used, the strength at the bore is not adequate, and if Heat Treatment B is used, there is not enough creep resistance at the high temperatures. Moreover, a surface flaw or crack can propagate rapidly under stress at temperatures above 900° F.
It was therefore generally thought desirable to use Heat Treatment A for the locations exposed to the higher temperature but at the same time have the tensile strength which Heat Treatment B can provide for the bore locations. No process exists, however, to develop different properties at different locations in the complex shape of a turbine rotor wheel or disk.
Prior U.S. patents of interest include U.S. Pat. No. 6,146,478; U.S. Pat. No. 5,846,353; U.S. Pat. No. 5,863,494; and U.S. Pat. No. 5,374,323. The '478 patent applies the INCO recommended Heat Treatment A with some modifications to large turbine disks. This heat treatment process will impart good rupture and crack growth resistance, but will have poor strength at low and intermediate temperatures. The '353 patent discloses modification of the composition for improved hot ductility at temperatures above 1300° F. The '494 patent modifies the process to achieve high strength at temperatures above 1300° F. The '323 patent describes a process of manufacturing a turbine disk using the Heat Treatment B but does not address the problem of creep and accelerated crack growth at temperatures above 900° F.
While the treatments described in certain of these patents (the '478, '353 and '494 patents) improve some properties at high temperatures, they do not also improve strength at low and intermediate temperatures.